Hard disk drives (HDDs) have become the preeminent mass storage medium in the field of digital information storage. In order to increase a magnetic field gradient to the surroundings of the main magnetic pole, the wrap around shield (WAS) is used in perpendicular magnetic recording. A perpendicular writer having a single main pole with a soft underlayer and a WAS can generate a strong magnetic recording field with a high magnetic gradient into the recording layer of a magnetic recording medium.
However, the recording magnetic field intensity experienced by adjacent tracks to the track being written becomes larger as magnetic media having higher track densities (TPI) are used. Consequently, tracks adjacent to the track being written are being eliminated or degraded, which is referred to as adjacent track interference (ATI). As field intensity continues to increase in modern magnetic recording systems, ATI is becoming quite great.
Moreover, another problem exists where written data may be eliminated, but instead of occurring in an adjacent track, the data elimination occurs at a far track, referred to as far track interference (FTI).
These problems may result from a magnetic domain that forms from the soft magnetic layer of the WAS and is based on a disclosure magnetic field therefrom. FIG. 9 shows an amount of degradation of the error rate caused by FTI. The horizontal axis denotes the track width direction position and the vertical axis shows the amount of degradation of the error rate by FTI. As shown in FIG. 9, the amount of degradation of the error rate increases relative to distance from a center crosstrack position.
In order to achieve higher density in a magnetic disk device, a perpendicular magnetic recording system is typically employed. In such systems, a WAS is used in the magnetic recording head in order to increase the magnetic field gradient around the main magnetic pole. This construction has a characteristic of producing a stronger recording magnetic field with a steeper gradient which can be applied to the recording layer of the magnetic medium to write data.
However, while higher track densities (TPI) may be achieved by increasing the density of recording, this results in the recording magnetic field intensity that is applied to adjacent tracks to become large, exacerbating the problem of adjacent track erasure due to ATI. Also, a problem arises where erasure of data occurs not only in adjacent tracks but also in tracks that are a few tracks further away due to FTI. It is believed that this is due to magnetic field leakage being produced from the magnetic domains that are formed by the soft magnetic film of the WAS. FIG. 9 shows the amount of degradation of the error rate produced by exemplary FTI. The horizontal axis shows the position in the track width direction and the vertical axis shows the amount of degradation of the error rate produced by FTI. It can be seen that there is a considerable amount of degradation of the error rate even in tracks that are a few tracks further away from the track being written.
As a countermeasure to the effects of ATI and FTI, systems have been developed (herein referred to as a “refreshment system”) for storing the information as to the position passed by the head, and the number of times the head passes, in respect of any given track on the HDD, and, taking into consideration this number of times and the position, re-recording the data in the corresponding track every time the head passes (recording) at least a fixed number of times, in fact the amount of information as to the position over which the head passes (recording) is restricted, due to limitations regarding memory, etc. The range over which this technique is to be applied is determined in terms of the track number and the track pitch.